I did JOGLE in 2001 and again in 2009, with different companions. The first time was on one of the original F-series Cannondales with a Lefty and fat slicks; the second was on a Bad Boy.
We had a rough idea of routes both times, but essentially made it up as we went along. Your approach may obviously vary, although I’d suggest that there aren’t that many options in some parts – e.g. northern Scotland. Sustrans routes were far better developed by the second time and we used them in parts, although they were slower and sometimes roughly surfaced.
It took 10 days on both occasions: staying in B&Bs, carrying our own luggage and obviously without a massively speedy mindset. If we wanted to stop, detour or see a sight, we did. We watched bad movies in local cinemas, took in some fairly average museums and had a top time.
I’ve done more touring since on more suitable bikes, but with a similar mentality. Britain is a varied and lovely island: be sure to take the time to enjoy it!